The present invention relates to a TV signal receiving system, and more particularly to a TV signal receiving system which, in order to tune a broadcasting frequency of a desired channel after receiving a TV signal, converts the TV signal into an intermediate frequency (hereinafter referred to as IF) higher than the maximum frequency in a TV broadcasting frequency band and then converts again the IF into another IF as high as that of a conventional TV system, thereby improving the rejection characteristics of image and IF components.
A heterodyne intermediate frequency converting system as shown in FIG. 1 is employed in a TV tuner for receiving a radio frequency (hereinafter referred to as RF) signal and selecting a desired channel in a conventional TV or VCR.
Such a tuner system uses variable capacitance diode devices to form an electronic type tuner which has no contact points which conventional mechanical type tuners have, thereby enabling to lengthen the life time of the tuner and to improve functions such as the remote control.
However, the conventional tuner system has a problem that the production cost and the size of the tuner are increased by using a large number of discrete device due to use of various coils because the tuner should be tuned using the characteristic variation and inductance value of the variable capacitance diode so called a varactor.
Furthermore, since an RF signal in 50 MHz to 900 MHz is tuned to a frequency of a desired channel and is directly converted to an IF signal of 45 MHz by using a direct down-conversion method in the conventional system, mixture and interference of an image frequency and IF components occur to deteriorate the tuner.
The characteristic of a conventional heterodyne demodulating system will be described below with reference to FIG. 1.
If an output carrier frequency of RF amplifier 3 is fc, an oscillating frequency of local oscillator 5 is f.sub.LO, and an output IF frequency of mixer 6 is f.sub.IF, the output IF frequency of mixer 6 in the heterodyne system becomes f.sub.IF =f.sub.LO -fc. Here, the value of f.sub.IF is 45 MHz in the NTSC TV system. At this time, if a frequency of an image component is f.sub.IM, the following equation is established. EQU f.sub.IM =fc.+-.2f.sub.IF.
If f.sub.IF frequency component affecting on the image component interferes in an input terminal, the component acts on a tuned channel signal as an interference signal to deteriorate TV picture quality.
In FIG. 1, reference numerals 2, 4, 7, 8 and 9 denoted in the drawing but undescribed in the specification represent an input tuning portion, an intermediate tuning portion, a band pass filter, an IF amplifier and a demodulator, respectively.
For instance, if a carrier frequency of a station is 150 MHz and an IF is 45 MHz in the existing NTSC system in case of adopting an direct down-conversion system, that is, fc=150 MHz and f.sub.IF =45 MHz, then an oscillating frequency f.sub.LO for RF demodulating the signals in local oscillator 5 is 195 MHz since f.sub.LO =f.sub.IF +fc, and an image frequency f.sub.IM is expressed as follows. EQU f.sub.IM =fc.+-.2f.sub.IF =150.+-.2.times.45=60 MHz/240 MHz.
The signal lies within the band of 50 to 900 MHz, that is, in-band of a TV signal, accordingly causes to be mixed with a signal around other channels to act as an interference signal.
Accordingly, rejection capability of such image components is greatly important in performance of a tuner system.
The reason why the image component exists within a TV band channel to act as an interference signal is that the value of the IF is 45 MHz which is much lower than a TV band. That is, in the conventional system using an IF of 45 MHz, an image frequency exists within .+-.90 MHz (2.times.45 MHz) of the TV broadcasting channel frequencies and mostly within the TV broadcasting frequency band.
Its main cause is that a tuned broadcasting signal is directly converted into a low frequency of 45 MHz.